February 27, 2008

Mechanisms of climate change

There are distinctly different and quite complex patterns of energy transfers at different latitudes of Earth. The global climate and energy budgets at latitudes are correlated so that if climate of one large area or globe as a whole changes then considerable changes in energy budgets of different latitudes may occur. Similarly if energy budget of one large area or globe as a whole changes then considerable changes in the global climate may also occur. Three major types of mechanisms of climatic change can be recognized viz. deterministic extrinsic, stochastic extrinsic and stochastic intrinsic mechanisms.

1. DETERMINISTIC EXTRINSIC MECHANISMSThese mechanisms of climatic change include those processes that are external to Earth’s climate system, are repetitive events i.e. cyclic events of definite periodicity and have forcing effect on global climate. Such mechanisms may be categorized as follows:

a) Mechanisms related to geometry of solar system

These include those processes that are due to cyclic variations in the geometry of Sun-Earth-Moon system.

i) Lunar cycle: It is the 14-day cycle of lunar tidal influences due to orbiting of Moon around Earth. The effect of tidal influences on the climate are observed as variations in cloudiness and rainfall.

ii) Milankovitch cycles: Three cycles of different periodicity have been recognized due to changes in Sun-Earth geometry.

100,000-year cycle: It is cyclic change in the shape of Earth’s orbit around Sun from nearly circular to markedly elliptical and back. At present the orbit is nearly circular and the difference between perihelion and aphelion is about 3.5%. In most elliptical condition of the orbit, this difference may be as large as 30%.

21,000-year cycle: It is the cycle due to wobbling of Earth’s axis of rotation. This wobble in 10,000 years time causes a shift to more extreme difference between summer and winter climates and hemispherical differences in solar constant.

40,000-year cycle: It is the cyclic change in the angle of inclination of Earth’s rotation within a range of 21.8o to 24.4o (presently being about 23.45o). Increase in the angle of inclination results in increased contrast between summer and winter climates.

Most important aspect of these cycles of variations in Earth-Sun geometry is that the distribution of solar irradiance of Earth, particularly between the two hemispheres, varies regularly during the period of each of these cycles. This results in changes in the radiation balance and consequently the energy balance, which affects the global climate. The above three cycles occur simultaneously but not synchronously (due to different periodicity) and, therefore, lead to a long and complex time series of climatic variation.

Considerable evidence has now accumulated in favour of the Milankovitch model in which he proposes that these three cycles lead to major ice ages on Earth. Whenever the summer insolation in the northern hemisphere decreases by 2% during these cycles, the ice cover of Earth spreads, particularly between the latitudes 50o to 70o N. This spread of ice cover results in increase in the albedo of Earth’s surface. This, in turn, leads to decreased absorption of solar radiation and consequent decline in the temperature. The spread of ice cover is also associated with decrease in atmospheric concentration of carbon dioxide. This decrease in carbon dioxide in atmosphere results in reduced atmospheric absorption of long-wave re-radiation from Earth’s surface, which results in further decrease in temperature. Thus, the initial spread of ice cover acts as trigger for increasing decline in temperature and consequently more and more spread of ice cover leading to major ice age.

b) Mechanisms related to solar radiant emittanceThese mechanisms also bring about climatic changes due to changes in solar irradiance of Earth but the cause of change in solar irradiance is actual cyclic variation in the output of radiation from Sun i.e. variation in solar radiant emittance.

Most important such mechanisms are Sunspot cycles of different periodicity. Sunspots are areas on the Sun’s surface which have temperatures lower than average surface temperature of Sun but have intense magnetic and solar activity. The number and area of sunspots is not constant but shows regular cyclic variation between some minima and maxima. Such sunspot cycles having periods of 11-years (occurrence of successive sunspot minima) and 22 to 23 years (double sunspot cycle associated with reversal of solar magnetic field) are quite important. It has been shown that solar constant varies by about 2% during such sunspot cycles. Other cycles having periodicity of 45, 80, 150, 200, 500, 1000 years have also been recognized. The impact of these cycles depends on their effectiveness in perturbing the global energy balance. Their effect may be realized as changes in pattern of atmospheric circulation.

2. STOCHASTIC EXTRINSIC MECHANISMSThese include processes that are external to Earth’s climatic system, have forcing effect on global climate but are not regularly repetitive but occur at irregular intervals i.e. are stochastic in nature. Most important such mechanism is volcanic eruptions. A very large quantity of volcanic ash and sulphuric acid droplets is injected into the Earth’s atmosphere, particularly into stratosphere during volcanic eruption. This addition of volcanic dust in the atmosphere reduces the amount of solar radiation reaching the Earth’s surface. This reduction in solar irradiance of Earth’s surface may result in reduced temperature, particularly during summers. Lamb (1971) has shown that for British Isles many of the coldest and wettest summers (e.g. during 1695, 1725, 1816, 1879, 1903, 1912 A.D.) occurred at times when volcanic dust content of stratosphere and atmosphere was high. Bray (1974) has proposed that volcanic activity may also have triggered the relatively recent glacial advances such as in periods of 5400-4700 B.C., 2850-2150 B.C. and 470-50 B.C. Possibly the injection of volcanic dust into atmosphere during a sensitive stage of Milankovitch cycles may hasten the approach of an ice age. Important considerations related to impact of volcanic eruption on global climate are:

a) Type and quantity of volcanic aerosols injected into atmosphere: Higher the amount and proportion of volcanic ash injected into the atmosphere, greater is the reduction in solar radiation reaching Earth’s surface.

b) Residence time of aerosols in stratosphere: If aerosols injected by volcanic eruption stay in stratosphere for sufficiently long time only then the decrease in solar irradiance will be realized as effect on the climate. The residence times of aerosols are inversely proportional to their size i.e. smaller the size, longer the residence time. Acid droplets in particular and ash to lesser extent may have residence times in stratosphere and atmosphere in the order of a few years.

c) Global air circulation: The particles injected into the atmosphere during volcanic eruption are spread over a range of latitudes by stratospheric winds depending on the pattern of global air circulation. According to Lockwood (1979), volcanic dust injected into equatorial zones of stratosphere is spread over whole of globe and perhaps is accumulated over polar ice caps. The dust injected in high latitudes does not spread to latitudes lower than about 300 N. Maximum effect of volcanic dust on global climate is due to this apparent tendency particles to accumulate and persist over high latitudes. This is because the effect of stratospheric particles on incoming solar radiation increases with latitude due to greater reflectance of radiation with increasing angle of incidence of solar beam.

3. STOCHASTIC INTRINSIC MECHANISMSThese mechanisms related to climatic change include those processes that originate within the climatic system of Earth i.e. are internal (intrinsic) processes. These mechanisms have forcing effect on climate and are not regular repetitive events but are probabilistic or stochastic in nature. Important such mechanisms of climate change are discussed below.

a) High-pressure and low-pressure atmospheric systems: Most familiar stochastic events are changes in synoptic weather patterns typified by movements of regions of high and low atmospheric pressure. Such pressure areas occur only for few days at most and their spatial and temporal distribution is controlled by the great flows of circumpolar vortex.

b) Anticyclonic and cyclonic systems: In each hemisphere of globe, there is considerable wind flow from west to east through most of the atmosphere. Majority of this flow is in middle latitudes. This circumpolar flow plays important role in creating and maintaining characteristic pattern of global climate. Variations in atmospheric pressure cause disturbances in this circumpolar flow and lead to climatic change. Such disturbances arise due to differences in degree of radiant heating of Earth’s surface. Low-pressure areas develop over colder and high-pressure areas over warmer regions. These high- and low-pressure areas in atmosphere in turn affect the velocity of circumpolar vortex. Local high-pressure areas accelerate while low-pressure areas deaccelerate the circumpolar flow. These processes thus result in development of high- and low-pressure atmospheric systems which in turn are moved around the globe by the circumpolar vortex. In general, high-pressure systems or anticyclones are maintained along the warm side of the upper wind and low pressure or cyclonic systems occur near the cold side of the main flow.

c) Meridonial flow system: The above described smooth zonal pattern of circumpolar vortex changes stochastically to a system of meanders from the middle latitudes to the pole and back again. This pattern is called meridonial flow system and may be long lasting. It exerts a blocking effect on the more typical pattern of climate or weather. The meridonial flow leads to periods of abnormally extreme weather with drought in one location and floods in another. The extremes of temperatures also occur in the same manner. Striking example of the effect of meridonial flow on climate was in summer of 1976 A.D. when climate of British Isles was particularly hot and dry while Greece and Turkey experienced wetter than average climate. It has also been suggested that increases in meridonial flow and resultant blocking of atmospheric flow might have caused or amplified the course of Little Ice Age during late 16th to early 18th centuries. The climate of this period varied considerably over whole of globe, observed as low precipitation during the Indian monsoons, expansion of polar ice caps, severe winters in Europe and yet a warmer climate in Siberia during the 17th century.

El Nino effect: The ocean currents also have very important relationship with characteristic global climatic pattern and any change in pattern of ocean currents results in considerable change in climate. Best known periodic variation of ocean currents is El Nino effect. It is the irregular fluctuation of ocean currents off the coast of Peru and Equador. Its recurrence time is stochastic between 3 to 8 years. Due to dominant, cold Peruvian current the coastline of Peru and Ecuador has very little precipitation and the adjacent sea is colder than the land. This climatic pattern is generally emphasized by trade winds from the southeast. During an El Nino event, these trade winds are replaced by northerly winds, which cause southward flow of warm equatorial water. This leads to heavy rainfall over the arid regions of Peru. The time scales of variations in the circumpolar vortex and of the El Nino event are quite short being in the order of few years only. Kutzbach (1976) has emphasized that intrinsic processes operating on time scales of millennia with interactions between the atmosphere, ocean currents and global ice may be important in understanding the regular occurrence of Ice Ages.